A moisture-permeable waterproof fabric, which has both moisture permeability capable of releasing vapor generated by sweating from the body to out of the clothes and waterproofing properties capable of preventing rainwater from penetrating into the clothes, and is heavily used as a clothing material for sporting and outdoor activities wherein comparatively many amounts of sweating are induced by exercise. As this type of moisture-permeable waterproof fabric, it is well known a layer wherein a resin film of a synthetic polymer is layered with a fabric cloth by a coating method or a laminating. Examples of such resin films include a nonporous film which is formed of a resin having moisture-permeability in itself and a porous layer, including a microporous layer, which does not have moisture-permeability in itself but is given moisture-permeability by forming many pores therein. Among them, a fibrous fabric layered with a porous film is excellent both in moisture-permeability, especially moisture-permeability as measured by the JIS L-1099 A-1 method (the calcium chloride method), and in waterproofing properties, and thus is favorably used.
While there are various methods for forming a porous film, a wet coagulation method is included as a typical method thereof. More precisely, a solution wherein a synthetic polymer consisting mainly of a polyurethane resin is dissolved in an organic solvent compatible with water is applied on a fibrous fabric or a releasable base material to coagulate by immersing them into water, whereby the organic solvent in the resin solution is replaced by water to form a film having many pores. It is known that performance of the moisture-permeable waterproof fabric is largely depending on the size and/or distribution of the pores. In order to improve performance and to stabilize in quality, it has been made a study to control the size and/or distribution of the pores.
In general, a porous film formed by a wet coagulation method has a honeycomb skin-core structure that is a two-layer structure having a inner layer called as a honeycomb layer and an outer layer called as a skin layer. There are very small pores having a pore diameter of not larger than 1 μm in the skin layer, and there are relatively large pores having a pore diameter of not larger than several dozen μm in the honeycomb layer, and they are communicated with each other. The honeycomb layer mainly contributes to moisture-permeability and the skin layer mainly contributes to waterproofing properties. The Patent Document 1 discloses a porous film having such a structure.
That is, the Patent Document 1 discloses a moisture-permeable waterproofing coated fabric having a porous resin layer of a polyurethane-based synthetic polymer on a fibrous fabric, wherein there are pores and micropores of a honeycomb skin-core structure in a cross-section of said resin layer and the longitudinal diameter of the pores present in the honeycomb layer is 1-40 μm, the area ratio thereof is 50-70% and the number thereof is (2-10)×103/mm2, the longitudinal diameter of the micropores present in the resin layer in whole is not larger than 1 μm, the area ratio thereof is 1-20% and the number thereof is (1-30)×105/mm2, and has a water pressure residence of 0.6-2.5 kgf/cm2 and a degree of moisture permeability of 8000-12000 g/m2·24 hours as measured by the calcium chloride method.
It also discloses, as a production method to obtain such a moisture-permeable waterproofing coated fabric, a method wherein subjecting a fibrous fabric to a wet type coating film-forming treatment after coating thereon a resin mixture solution containing a polyurethane-based synthetic polymer containing not less than 1 wt %, preferably not less than 3 wt % of inorganic particles having an average particle diameter of not larger than 1 μm and an N,N-dimethylformamide adsorption rate of not less than 200 mL/100 g and a polyurethane-based synthetic polymer having a different coagulation value from said polyurethane-based synthetic polymer.
However, since the above moisture-permeable waterproofing coated fabric has relatively large pores present in the honeycomb layer and they dominate a large area ratio (consequently, a large volume ratio) thereof, it is insufficient in waterproofing properties, and thus it is necessary to form another non-porous film thereon in order to obtain sufficient waterproofing properties at practical level.
The Patent Document 2 discloses a moisture-permeable waterproof fabric comprising a microporous film of a urethane resin formed on a fibrous fabric, wherein the microporous film contains preferably 10-100 parts by mass, more preferably 40-70 parts by mass of calcium carbonate particles based on 100 parts by mass of the urethane resin. The microporous film of the moisture-permeable waterproof fabric has fine pores having not larger than around 1 μm on its surface and has pores of 10-50 μm inside.
However, said moisture-permeable waterproof fabric, since having relatively large pores of a honeycomb skin-core structure in the microporous film, also has similar problems as the fabric described in the Patent Document 1.
For the purpose of obtaining a well-balanced combination of moisture-permeability and waterproofing properties fabric, in general, the pores in a porous film are required to be smaller, to be increased in number and to be uniform. For example, the Patent Document 3 discloses a porous sheet material used by means of attaching to or burying in a part of a base material such as a nonwoven fabric, a woven fabric and a knitting fabric, which is obtained by mixing 100 parts by weight of a polar solvent-soluble polymer material with 30-500 parts by weight of a powdery inorganic material and has interconnected pores having an average pore diameter of not larger than 10 μm in the sheet material. It is described that the mixing ratio of the powdery inorganic material is preferably 50-300 parts by weight, more preferably 100-300 parts by weight, and is explained that, by mixing the powdery inorganic material in this range, downsizing of pores and uniformizing of the pore diameters would be more accelerated and an average pore size of the interconnected pores would be 1 μm or smaller.
However, in order to downsize the pore diameter, it would be necessary to mix large amounts of the powdery inorganic material, which might cause deterioration of strength, in particular, tensile strength of the porous sheet material thus obtained, and in the result, problems such as deterioration of waterproofing properties might occur.
Regarding the above problems, the Patent Document 4 disclose a method of producing a moisture-permeable waterproof fabric wherein a synthetic polymer solution consisting mainly of a polyurethane resin containing 15-50 mass % of inorganic fine powder having an average particle diameter of not larger than 0.100 μm and the adsorption amount of N,N-dimethylformamide of not less than 300 mL/100 g based on the total solid content is applied on a fibrous fabric, and subsequently the product thereof is immersed in a N,N-dimethylformamide aqueous solution having the concentration of 10-70%, and then is subjected to hot-water washing and drying.
According to the above method, a moisture-permeable waterproof fabric having a resin layer of substantially single layer structure on a fibrous fabric wherein micropores having the pore diameter of preferably 3.00 μm or less are formed in said resin layer, having a degree of moisture permeability of 7000-12000 g/m2·24 hours as measured by the calcium chloride method and having a water pressure resistance of 120−300 kPa can be obtained.
However, since the coagulated liquid contains a large amount of N,N-dimethylformamide (e.g., 40% in Example), it would be necessary to give consideration to the working environment and the global environment at the time of manufacturing moisture-permeable waterproof fabrics and disposing the coagulated liquid, which would cause an increase in costs.